Paste for welding ceramics to metals and method for producing a welded joint

ABSTRACT

The invention proposes a paste for welding ceramic materials to metallic contact surfaces or metals, which contains a noble metal, an aluminum compound, a silicon compound, and a compound selected from the group of the barium compounds, calcium compounds, and magnesium compounds. Along with these, a binder, a solvent, a softener, a thixotropic agent, and a dispersing agent can also be added to the paste. A paste of this kind is used to produce a welded joint between ceramic materials and a metallic contact surface, the paste preferably being printed onto the ceramic material. The ceramic material is then sintered with the paste, and then the metallic contact surface or a metal is welded on.

BACKGROUND OF THE INVENTION

The invention relates to a paste for welding ceramic materials tometallic contact surfaces or metals according to the species defined inthe principal claim, as well as a method for producing a welded joinbetween ceramic materials and metals using this paste.

It is commonly known that metallic connecting contacts can beimplemented on ceramic substrates by way of bonding, welding, orsoldering methods. The contacts and contacting methods known so far arenot suitable for high mechanical and thermal stresses, however, sincethe adhesion between the connecting contact and ceramic substrate isoften insufficient. Plug contacts have hitherto been used in such cases.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to develop a paste, anda method that can be carried out therewith, for welding metallic contactsurfaces or metals to ceramic materials, in which context the pastewithstands high thermal and mechanical stresses so as thereby to avoidthe use of cost-intensive standard plug contacts.

ADVANTAGES OF THE INVENTION

The paste according to the present invention having the characterizingfeatures of the principal claim, and the method carried out therewithfor welding metallic contact surfaces to ceramic materials, has theadvantage, as compared to the existing art, of making possible directwelding of metallic contact surfaces, for example lead frame materials,to ceramic standard materials via an intermediate layer made of thepaste; the weld, while having good electrical properties, at the sametime withstands high thermal and mechanical stresses, so that plugcontacts hitherto used can be omitted. A considerable reduction in thecost of manufacture is thus also achieved.

The paste according to the present invention thus makes possible insimple fashion, for example, electrical contacting to sensor elementsusing welding methods, in which context metallic materials are weldedonto standard substrates and the paste according to the presentinvention acts as an adhesion layer. The temperature resistance of thewelded join is guaranteed up to at least 500° C.

Advantageous developments of the invention are evident from the featuresrecited in the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

As a result of further added paste components such as a binder, asoftener, a solvent, a dispersing agent, and a thixotropic agent, it ispossible in particular to influence the processability of the pasteaccording to the present invention very advantageously in terms ofviscosity and handling, so that it is also suitable, for example, as aprinting paste or for application by way of a ductor, and can thuseasily be integrated in terms of process engineering into existingproduction techniques.

It is also advantageous if, prior to the sintering of the ceramicmaterial and paste, a platinum conductive paste is additionally printedonto the paste that has been applied onto the ceramic material and, inparticular, dried. The electrical conductivity of the welded join canthereby be improved, in particular if the paste according to the presentinvention contains low concentrations of noble metal.

EXEMPLARY EMBODIMENTS

The paste according to the present invention contains at least one noblemetal, at least one aluminum compound, at least one silicon compound,and at least one compound selected from the group of the bariumcompounds, magnesium compounds, and calcium compounds.

Moreover, in an advantageous development of the invention, a binder, asoftener, and a solvent are additionally added for simplified handlingand production of the paste. In addition, if necessary, a thixotropicagent and a dispersing agent are incorporated into the paste ifnecessary. Platinum, palladium, silver, or gold is especially suitableas the noble metal.

Suitable barium compounds are, in particular, barium oxide, bariumcarbonate, barium hydrate, barium acetate, or another organic bariumcompound; it is very advantageous if the barium compound, duringsubsequent drying and/or sintering of the paste according to the presentinvention together with the ceramic material and the metallic contactsurface, decomposes or is converted into barium oxide as completely andwith as few residues as possible. Also particularly advantageous is theaddition of barium oxide as the barium compound to the paste accordingto the present invention, since, for example, vigorous gas evolution orthe release of water is disadvantageous in the context of welding.

The barium compound in the paste according to the present invention canbe entirely or partially replaced by a magnesium compound and/or acalcium compound; MgO, CaO, MgCO₃, CaCO₃, the hydroxides of calcium ormagnesium, and organic magnesium or calcium compounds are particularlysuitable for this purpose, since after drying and/or sintering they veryadvantageously, similarly to the requirements for the barium compound,are present at least largely as corresponding metal oxides.

Silicon dioxide, an organic silicon compound, or a silicon-oxygencompound is suitable as the silicon compound; it is again veryadvantageous if the silicon compound, during subsequent drying orsintering of the paste according to the present invention together withthe ceramic material and the metallic contact surface, decomposes or isconverted into silicon dioxide as completely and with as few residues aspossible. The addition of silicon dioxide, kaolin, kaolinite, orMg₂(SiO₄) (forsterite) as the silicon compound is particularlypreferred.

Aluminum oxide (Al₂O₃), an organic aluminum compound such as aluminumformate, or aluminum hydroxide (Al(OH)₃ or AlO(OH)) is suitable as thealuminum compound; here again, as in the case of silicon and barium, itis very advantageous if the aluminum compound, during subsequent dryingor sintering of the paste according to the present invention togetherwith the ceramic material and the metallic contact surface, decomposesor is converted into aluminum oxide (Al₂O₃) as completely and with asfew residues as possible. Direct addition of aluminum oxide (Al₂O₃) asthe aluminum compound is particularly preferred.

A paste using platinum, silicon dioxide, aluminum dioxide, and bariumoxide has proven to be very particularly preferred in terms of adhesion,economy, and simple handling.

Possible compositions of the paste according to the present inventionare reproduced in Examples 1 through 3 of embodiments. The use of therelatively small quantities of barium compound, such as barium oxide orbarium carbonate (which can be entirely or partially replaced by acalcium or magnesium compound), and of the silicon compound such assilicon dioxide, has proven to be an essential feature of the inventionthat is claimed.

EXAMPLE 1

60.8 wt % platinum

16.7 wt % aluminum oxide

0.4 wt % silicon dioxide

1.6 wt % barium oxide;

the following are additionally added:

4.0 wt % polyvinyl butyral as binder

1.4 wt % dibutyl phthalate as softener

15.1 wt % Butyl Carbitol as solvent.

EXAMPLE 2

60.7 wt % platinum

15.2 wt % aluminum oxide

0.5 wt % silicon dioxide

0.9 wt % barium oxide

the following are additionally added:

4.3 wt % polyvinyl butyral as binder

1.6 wt % dioctyl phthalate as softener

16.8 wt % Butyl Carbitol as solvent.

EXAMPLE 3

38.2 wt % platinum

39.7 wt % aluminum oxide

1.2 wt % silicon dioxide

0.4 wt % barium oxide

the following are additionally added:

4.0 wt % polyvinyl butyral as binder

1.4 wt % dibutyl phthalate as softener

15.1 wt % Butyl Carbitol as solvent.

Depending on the solvent, many wetting and dispersing agents known perse are suitable as dispersing agents for addition to the pastesdescribed in Examples 1 through 3. In particular, the use of productsunder the trade names Diperplast or Disperbyk of BYK-Chemie GmbH,D-46462 Wesel, Germany, which can be added as necessary at aconcentration of 0.2 to 6 wt %, has proven advantageous. Polyvinylbutyral or ethyl cellulose, for example, is suitable as the thixotropicagent for addition to the pastes described in Examples 1 through 3. Itcan be added at a quantity of 0.5 to 25 wt %.

The noble metals and the aluminum or silicon compound or the compoundfrom the group of the barium compounds, calcium compounds, or magnesiumcompounds are advantageously used in the form of very finely groundpowders whose particle sizes lie in the range from 0.1 μm to 50 μm.

The specific surface of the noble metal powders is advantageouslybetween 1 and 10 m²/g, and that of the aluminum compound powder isadvantageously between 5 and 15 m²/g. A platinum powder having aspecific surface of approx. 3 m²/g and an aluminum oxide (Al₂O₃) powderhaving a specific surface of approx. 10 m²/g are particularly preferred.

All the components of the paste are mixed in a manner known per se, andprocessed into a homogeneous mass. This is then applied, in particularby spreading, printing, or brushing, onto the ceramic material that isto be welded to the metallic contact surface, and is dried if necessary.

The metallic contact surface, for example in the form of a lead frame ora thin foil, is then applied onto the dried paste and is sintered atapproximately 1500-1550° C., in a co-firing process, together with theceramic material and the dried paste. A metallic contact can then bewelded in a manner known per se onto the metallic contact surface, thepaste on the one hand acting as an adhesion promoter between the metalcontact surface and ceramic material, and on the other hand serving atleast locally as an electrically conductive intermediate layer.

Alternatively, after the paste, processed into a homogeneous mass, hasbeen applied onto the ceramic material and dried, sintering of theceramic material with the paste present thereon can also first beperformed, this sintering being accomplished, as above, using theco-firing method at approximately 1500-1550° C. Then the metalliccontact surface, for example in the form of a lead frame or a thin metalfoil, is applied onto the paste sintered to the ceramic material, and iswelded in a manner known per se so as to create a metallic contact, thepaste on the one hand serving as an adhesion promoter between themetallic contact surface and ceramic material, and on the other handserving at least locally as an electrically conductive intermediatelayer between the metal and the ceramic material welded thereto.

In a development of the two methods described above, it is moreoverpossible for a further platinum conductive paste, known per se, first tobe applied, before sintering and welding, onto the dried paste appliedonto the ceramic material. Both pastes are then preferably dried againbefore being processed further in the two variant methods describedabove.

The modified methods using the platinum conductive paste have theadvantage that the paste according to the present invention can beprocessed with a lower concentration of noble metal without losing theelectrical conductivity between the welded-on metal and the ceramicmaterial. The platinum conductive paste thus serves to form anelectrically conductive layer.

A substrate made of aluminum oxide, silicon carbide, or zirconiumdioxide is especially suitable as the ceramic material.

What is claimed is:
 1. A paste for welding ceramic materials to metalliccontact surfaces or metals, comprising a paste including at least onenoble metal, at least one aluminum compound, at least one siliconcompound, and at least one compound selected from the group consistingof barium compounds, calcium compounds, and magnesium compounds, whereinthe paste contains 5 to 60 wt % of an aluminum compound, 0.2 to 5 wt %of a silicon compound, 0.2 to 5 wt % of a compound selected from thegroup consisting of barium compounds, calcium compounds, and magnesiumcompounds, and 20 to 80 wt % noble metal.
 2. The paste as defined inclaim 1, further including a polyvinyl butyral binder.
 3. The paste asdefined in claim 1, further including a dibutyl phthalate or dioctylphthalate softener, or a Butyl Carbitol solvent.
 4. The paste as definedin claim 1, further including a polyvinyl butyral or ethyl cellulosedispersing agent or thixotropic agent.
 5. The paste as defined in claim4, wherein the dispersing agent is present at a concentration of 0.2 to6 wt % and the thixotropic agent is present at a concentration of 0.5 to25 wt %.
 6. The paste as defined in claim 3, wherein the solvent ispresent at a concentration of 9 to 30 wt %.
 7. The paste as defined inclaim 3, wherein the softener is present at a concentration of 0.3 to 5wt %.
 8. The paste as defined in claim 1, wherein the paste contains 1to 10 wt % binder.
 9. The paste as defined in claim 1, wherein the noblemetal, the aluminum compound, the silicon compound, and the compoundselected from the group consisting of barium compounds, calciumcompounds, and magnesium compounds are introduced as very finely groundpowder, having a particle size of 0.1 μm to 50 μm.
 10. A paste forwelding ceramic materials to metallic contact surfaces or metals,comprising a paste including at least one noble metal, at least onealuminum compound, at least one silicon compound, and at least onecompound selected from the group consisting of barium compounds, calciumcompounds, and magnesium compounds, wherein the noble metal, thealuminum compound, the silicon compound, and the compound selected fromthe group consisting of barium compounds, calcium compounds, andmagnesium compounds are introduced as very finely ground powder, havinga particle size of 0.1 μm to 50 μm, and wherein the specific surface ofthe powder of the silicon compound is between 5 and 15 m²/g, and thespecific surface of the noble metal powder is between 1 and 10 m²/g. 11.The paste as defined in claim 1, wherein the noble metal is platinum.12. The paste as defined in claim 1, wherein the barium compound is abarium oxide, barium carbonate, barium acetate, barium hydroxide, or anorganic barium compound.
 13. The paste as defined in claim 1, whereinthe calcium compound is a calcium oxide, calcium carbonate, calciumhydroxide, or an organic calcium compound.
 14. The paste as defined inclaim 1, wherein the magnesium compound is a magnesium oxide, magnesiumcarbonate, magnesium hydroxide, or an organic magnesium compound. 15.The paste as defined in claim 1, wherein the aluminum compound is Al₂O₃,Al(OH)₃, AlOOH, or an organic aluminum compound.
 16. The paste asdefined in claim 1, wherein the silicon compound is silicon dioxide,kaolin, kaolinite, Mg₂(SiO₄), an organic silicon compound, or asilicon-oxygen compound.
 17. A method for producing a welded joinbetween ceramic materials and metals or metallic contact surfaces,comprising the steps of applying the paste according to claim 1 onto aceramic material and sintering the ceramic material with the paste,wherein sintering is performed at a temperature of 1450° C. to 1600° C.,and wherein, after sintering, a metallic contact surface, a metal foil,or a metal is welded onto the ceramic material equipped with the paste.18. The method as defined in claim 17, wherein the paste is dried afterapplication.
 19. The method as defined in claim 17, wherein the paste isprinted onto the ceramic material.
 20. The method as defined in claim17, further comprising the step of printing a platinum conductive pasteat least locally onto the paste after the paste has been applied. 21.The method as defined in claim 20, further comprising the steps ofdrying and sintering the ceramic material equipped with the paste andthe platinum conductive paste.
 22. The method as defined in claim 21,wherein after sintering, a metallic contact surface, a metal foil, or ametal is welded onto the ceramic material equipped with the paste andthe platinum conductive paste.